Physical Machine Memory Vs. Virtual Machine Memory

[njtreker]

Hello, We have a farm of Vsphere servers. When allocating memory to a virtual machine is there a different metric I should be using to determine how much memory to assign the VM. For example, Microsoft reccomends assigning 4 gigs of RAM to a server running Windows 2008 R2 and SQL 2008 R2. Becuase it's a virtual machine should I be assigning less memory to this VM? Should I just be assigning memory to systems based on software reccomendations? Is there a formula I should be using instead? Thanks in advance.

[weinstein5]

Welcome to the Community - I always take the recommendation from the software manufacturer to start with particularly if I do not have any actual perfromance data - I then monitor the VM to see if the set memory is approriate and then raise or lower as needed - this is one of the strengths of a virtual machine -

[njtreker]

Thank you! With regards to sizing memory, I have always built in a 40 - 45% cushion for resources to allow for spikes and additional temporary overhead. I realize with a VM I can run it a little tighter. Would you say 35 - 40 % free memory in Windows to allow for spikes, peaks, and additional programs to be run on an occasional basis is reasonable? I am just trying to get a handle if I should be allocating memory to Windows differently that I would if this was a physical host.

Thanks.

[mittim12]

It really comes down to what you are comfortable with in your environment.   If the VM spikes during the middle of the night you may not care and can leave it at a lower number.   If it's user facing and spikes during the day you may want that cushion.  

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